1. Field of the Invention
The invention relates to the field of multiple-input, mutliple output (MIMO) systems for multi-packet reception (MPR) in which collisions are resolved in physical layer using Alamouti coding.
2. Description of the Prior Art
The use of multiple-packet reception (MPR) in wireless networks is known to improve throughput especially in high traffic conditions. The lack of synchronization among the nodes in random access systems introduces significant challenges with respect to the adoption of MPR in the physical layer (PHY) of the open systems interconnection (OSI) model and the link layer or media access control (MAC) design for systems using MPR. An instantiation of PHY connects a link layer device to a physical medium such as an optical fiber or copper cable. For example, in a wireless network the PHY portion consists of the RF, mixed-signal and analog portions, that are often called transceivers, and the digital baseband portion that place high demand on the digital signal processing (DSP) and communication algorithm processing, including channel codes. It is common that these PHY portions are integrated with the media access control (MAC) layer in system-on-a-chip (SOC) implementations.
Layered OSI architecture is the de facto standard of operation in wired networks, namely a seven layer design comprised of the physical layer (PHY), data link layer, network layer, transport layer, session layer, presentation layer and application layer. The data link layer is comprised of two sublayers, namely the logical link control sublayer (LLC) and the media access control sublayer (MAC). LLC multiplexes protocols running atop the data link layer, and optionally provides flow control, acknowledgment, and error notification. The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the originator and recipient machines. The MAC sublayer is below the LLC sublayer and determines who is allowed to access the media at any one time. Other times it refers to a frame structure with MAC addresses inside. In wired networks, isolated-per-layer optimization techniques have been successfully and practically applied to improve network performance. However, applying per layer optimization techniques is of limited value in wireless networks due to openness of transmission media. Instead, cross-layer optimization techniques have gained widespread use in the wireless network design methodologies.
MAC protocol design within the data link layer is traditionally undertaken independently of the PHY layer. Design of the PHY layer in this case assumes that the PHY layer is incapable of detecting colliding packets and hence simultaneous transmissions always fail. In reality, multi-user detection (MUD), successive interference cancellation (SIC), and code division multiple access (CDMA) are examples of multiple packet reception (MPR) techniques wherein the PHY layer can detect more than one packet at a time. Hypothetically, detecting two packets at a time can double the throughput of a network. With MPR techniques, collisions are resolved in the PHY as opposed to the MAC layer and simultaneous transmissions are possible. While resolving collisions at the PHY layer can simplify MAC design, because the PHY can detect a number of simultaneous packet receptions, the MAC is still required to handle higher order collisions. This will remove the separation of MAC and PHY layers but allows for the enhancing of performance.
In practical networks, it is almost impossible to have a fully synchronized reception from physically separated clients. At the very least, an asynchrony by a fraction of a symbol-transmission duration is always expected due to different propagation delays. Therefore, MPR methods must detect multiple asynchronous transmissions. Asynchronous MPR methods are mostly complicated. Direct sequence-code division multiple access (DS-CDMA) is one of the simple asynchronous MPR methods. However, cross-layer design techniques relying on CDMA suffer from exhaustive code search overhead. One solution has been to assign codes to different packet types instead of different users to design a MAC algorithm for multi-hop ad-hoc networks. Unfortunately, the power control overhead associated with DS-CDMA reduces the practical value of the algorithm. Under some signal-to-interference-plus-noise ratio (SINR) and timing constraints, the use of a message-in-message (MIM) scheme enables concurrent transmissions. A shuffle algorithm has been proposed to centrally schedule transmissions by different interfering access points (APs). This algorithm takes into account the timing requirements of MIM and schedules transmissions from different APs to comply with that in a manner that enables successful simultaneous transmissions. Another algorithm which targets solving the collision problem in the PHY is ZigZag decoding. Utilizing an iterative algorithm, ZigZag decoding resolves two similar consecutive collisions. Although the algorithm does not require any central decision making unit, it requires the observation of multiple collisions between two packets before decoding such collided packets.
Other simple MPR techniques are those based on multiple-input multiple-output (MIMO) systems. Switching to higher frequencies increases the feasibility of having multiple antenna users. MIMO communications enhances the performance of a wireless communication system in a number of different ways. The available diversity gain with space-time codes (STCs) enhances the link quality and can be used to increase the data rate by means of using denser signal constellations. By spatial multiplexing, for example using vertical-Bell Laboratories layered space-time (V-BLAST), several parallel independent data streams can be sent simultaneously to increase the throughput. Beamforming concentrates the transmission energy in one direction in order to increase SNR and range. On the other hand, interference nulling prevents reception from certain directions and reduces the level of interference sensed from other transmissions.
Recently, MPR methods have been developed for MIMO systems. For example, transmit antennas are not required to be on a single node in order for V-BLAST receiver to work. Therefore, multiple streams of data coming from different sources can be separated at a multiple-antenna receiver. An MPR method based on STC has also been developed for two users with two transmit antennas. Independent space-time coded streams are separable at a multiple-antenna receiver by preserving some degree of diversity.
Prior designs of a MAC for WLANs without hidden terminals have been proposed. The MAC algorithm is the same as IEEE 802.11 with request-to-send and clear-to-send (RTS/CTS) signaling except that it has additional receiver address fields in CTS and acknowledgment (ACK) frames to grant permission for the transmission of more than one node at a time. The design assumes that all nodes are single-antenna nodes and a multiple-antenna AP utilizes a multi-user dimension (MUD) method to detect different data streams. However, the major drawback of the design is that it completely ignores the hidden terminal problem. More importantly, the MPR method cannot achieve good bit error rate (BER) performance and loses diversity as the number of streams to be decoded increases. When the MPR method has a high BER, long data packets will be dropped with a high probability because of the error. This degrades the throughput of the MPR system in comparison to traditional systems. The latter is due to the fact that the overhead is in the order of long data packets as opposed to short RTS packets wasted by collisions.
An important benefit of employing STC-based MPR is the providing of a high diversity order compared to other MIMO-MPR methods. An MPR-aware MAC for a WLAN has been developed based on slotted ALOHA net. The PHY layer design is a combination of spatial multiplexing and STC-based MPR. All nodes as well as the AP are two-antenna nodes. In this method, each packet is broken into two equal-length sections and sent through each antenna. The AP examines the received signal in order to find the number of simultaneously received packets. For single-packet receptions, V-BLAST is used. For double packet receptions, a retransmission follows and STC-based MPR is employed. While this MPR-aware MAC improves the performance in comparison to slotted ALOHA net, it yet again ignores the problem of hidden terminals and further assumes the nodes to be perfectly synchronized.